Fluorocarbon polymers ("fluoropolymers") are durable thermoplastic substances which are resistant to chemicals, oxidation, moisture, weathering, ozone and ultraviolet radiation. These polymers include polytetrafluoroethylene (PTFE), chlorotrifluoroethylene, fluorinated ethylene-propylene polymers, polyvinylidene fluoride, and hexafluoropropylene among others. Their "non-stick" chemical resistant surfaces have found uses in electrical components, chemical processing equipment, and in coatings for cooking utensils, pipes and gaskets. However, the very characteristic which makes these polymers useful--the "non-stick" surface--also makes the formation of products having a fluoropolymer surface on a different substrate, for example, a fluoropolymer coating on a metal cooking utensil, very difficult since such polymer surfaces cannot be conveniently bonded. Modification of the fluoropolymer surface to render it bondable to other materials is thus required. This is commonly done by etching the surface of the fluoropolymer component before bonding. This "etching" has been performed in the past by reacting the surface with a suitable fluid metal reactant as disclosed in U.S. Pat. No. 2,789,063 to Purvis, et al. The Purvis method replaces fluorine in the surface layer with the metal reactant, making the surface more hydrophilic. Alkali metals, alkaline earth metals, manganese and zinc can be used in the Purvis method to react with fluoropolymer surfaces at high temperature where the metal is a vapor, a metal hydride, or in solution in an amine-type solvent (including liquid ammonia). The systems described by Purvis, et al. are effective but difficult to handle. However, sodium solutions in liquid ammonia are still used today.
The most important chemical etchants used with fluoropolymers employ the solution of a complex of an alkali metal with a polyaryl hydrocarbon solvent such as sodium/naphthalene in dialkyl glycol ether.
All of the above methods have been applied to etching dense, "white" fluoropolymer surfaces. Etching is usually accomplished by dipping the clean fluoropolymer part into the etching solution for a brief time, usually one minute or less, after which it is washed and dried. However, such methods have not been useful for etching fluoropolymer fibers, in particular PTFE fibers, which require, in forming composite materials, a strong particle-matrix bond. PTFE fiber is spun from a viscose rayon matrix containing small particles of PTFE. This fiber is heated to fuse the PTFE and burn out the viscose. The result is a brown, "natural" fiber which has most of the desirable properties of white bleached fiber, including low surface energy (non-stick). Bleaching is a costly procedure, so the "natural" fiber is a less expensive alternative.
However, attempts to etch small diameter chopped, natural fibers by prior art procedures have failed, making difficult the production of durable composite materials using such fibers. Thus, a need exists for a procedure by which such fluoropolymer fibers may be etched and thus rendered bondable.